Ten questions concerning water quality in building hot water systems

Hot water systems - water heaters and connected downstream plumbing and fixtures - are at the center of the water-energy nexus. They are ubiquitous and deliver an essential service but are subject to water quality degradation and health risks, scalding risks, energy consumption and greenhouse gas production, and water wastage. While the focus of this paper is on water quality, hot water quality is intimately connected to hydraulics, system layout, water conservation, energy conservation, and user outcomes. Despite their importance, hot water systems are less studied than drinking water production and transmission systems. Better knowledge about hot water will help professionals who design, install, and operate the systems; system users to manage competing risks; and researchers to identify opportunities for innovation. The ten questions posed are focused on the United States. The questions were developed during a workshop that brought together experts in all aspects of hot water systems to identify and prioritize research and innovation opportunities for hot water systems. The questions are ordered to first provide a general description of hot water systems and the chemical and microbiological processes that occur within them, then to discuss the state-of-the-art in modeling and informatics (both critical given the wide diversity in hot water systems), next to explore user and usage aspects of systems and the importance of user-focused design, and finally to identify and prioritize hot water system research and innovation opportunities.